1. Field of the Invention
This invention relates to apparatus and a method for testing valves, and more particularly, for set point and seat leakage testing of pressure relief valves with selected fluids in selected phases.
2. Background Information
Set point testing of relief valves is required by the American Society of Mechanical Engineers (Sections III, VIII) to assure that vessels, boilers and pipelines are not overpressurized. To accomplish this, two schemes are generally used, viz:
(1) one approach is to remove the valve to be tested from the protected equipment and test it in a shop with water, nitrogen or steam, as available. Nitrogen is commonly used because it is generally available.
(2) another technique is to leave the relief valve in place on the protected equipment and to install on top of the valve an assist device, typically pneumatic or hydraulic powered, that will aid the valve to open. By monitoring system conditions in the output of the assist device, it is possible to back calculate the set point of the relief valve.
While the above schemes are generally acceptable, there is always a desire among maintenance and operations people, to test relief valves for set point and leakage in the actual medium and conditions in which they are used to confirm their operational readiness. For example, set point testing of relief valves used in steam service with nitrogen is permissible, but there is no reliable correlation that converts the test results in one medium to another. Similarly, the use of power assist devices introduces some uncertainty in data reduction in terms of the interaction of the displacements of many interacting valve parts on the estimated set point. Since the set point is back calculated when power assist devices are used, the accuracy of the final result depends upon the precision of the recorded variables. Thus, while the present test schemes are acceptable, they can still be improved upon.